High-purity carbon nanotubes were prepared in sizes ranging from 7 to 35 nm. The electronic structure and optical limiting characteristics of the tubes were examined by transmission electron microscopy, X-ray diffraction analysis, and ultraviolet photoemission, X-ray photoelectron and optical spectroscopies. The interplanar distance, work function, Fermi edge, core-level shift spectra, and plasma excitation of the carbon nanotubes were compared with those of graphite nanotubes.